Aromatic hydrocarbons, particularly benzene, toluene, ethylbenzene and xylenes, are important commodity chemicals in the petrochemical industry. Currently, aromatics are most frequently produced from petroleum-based feedstocks by a variety of processes, including catalytic reforming and catalytic cracking. However, as the world supplies of petroleum feedstocks decrease, there is a growing need to find alternative sources of aromatic hydrocarbons.
One possible alternative source of aromatic hydrocarbons is methane, which is the major constituent of natural gas and biogas. World reserves of natural gas are constantly being upgraded and more natural gas is currently being discovered than oil. Because of the problems associated with transportation of large volumes of natural gas, most of the natural gas produced along with oil, particularly at remote places, is flared and wasted. Hence the conversion of alkanes contained in natural gas directly to higher hydrocarbons, such as aromatics, is an attractive method of upgrading natural gas, providing the attendant technical difficulties can be overcome.
A large majority of the processes currently proposed for converting methane to liquid hydrocarbons involve initial conversion of the methane to synthesis gas, a blend of H2 and CO. However, production of synthesis gas is capital and energy intensive and hence routes that do not require synthesis gas generation are preferred.
A potentially attractive route for upgrading methane directly into higher hydrocarbons, particularly ethylene, benzene and naphthalene, is dehydroaromatization or reductive coupling. This process typically involves contacting the methane with a catalyst comprising a metal or metal carbide, such as molybdenum carbide, supported on a zeolite, such as ZSM-5, at high temperature, such as about 600° C. to about 1000° C., and low pressure, typically about 100 kPa to about 600 kPa. However, these conditions also favor build-up of carbon and other non-volatile materials, collectively referred to as “coke”, on the catalyst resulting in rapid loss of activity and potentially undesirable selectivity shifts, as well as loss of valuable feedstock. As a result, the catalyst is required to undergo frequent transfer, often every few minutes, between a reaction cycle, in which the catalyst effects methane conversion and accumulates coke, and a regeneration cycle, in which the coke is removed from the catalyst.
Thus the successful application of reductive coupling to produce aromatics on a commercial scale requires the development of a regeneration process that is not only effective at removing coke but also has minimal adverse affect on the metal-containing catalyst.
Currently, most methane dehydroaromatization processes propose the use of regeneration in the presence of an oxygen-containing gas since this is known to be very effective at coke removal. For example, U.S. Patent Application Publication No. 2007/0249879 discloses a process for converting methane to aromatic hydrocarbons over a catalyst comprising molybdenum, tungsten, zinc and/or rhenium in metallic or carbide form on a support, such as, ZSM-5, in which the coked catalyst is regenerated in an oxygen containing gas which may also contain carbon dioxide and/or nitrogen such that the oxygen concentration of the regeneration gas is from about 2 wt % to about 10 wt %.
Likewise, WO 2009/076005 teach a method of dehydroaromatizing methane with a catalyst comprising montmorillonite, a non-zeolitic molybdenum compound such as molybdenum oxide, and at least one zeolite that comprises at least one element selected from Cr, Mo, Fe, Co, Ni, Zn, Re, Ru, Rh, Pd, Os, Ir, Pt, W, and V. After deactivation, it is taught that the deactivated catalyst is re-activated via oxidation by exposure to air or some other suitable O2-containing gas stream or a less severe regeneration such as using H2 or a mixture of CO/CO2 to achieve a low oxygen concentration. A preferred mixture of CO/CO2 has a volumetric ratio of 1:1.
However, the above approaches have problems. For example, depending on the composition of the catalyst, regeneration in an oxidative environment can lead to a variety of unwanted ancillary results. For example, the metal on the catalyst may be converted from a catalytically active elemental or carburized state to a less active oxidized state. Also, following regeneration, the catalyst may exhibit enhanced activity for coke deposition and related hydrogen generation. In particular, with a molybdenum-containing catalyst on an aluminosilicate support, it is found that oxidative regeneration can cause rapid and permanent deactivation of the catalyst, due to effect such as production of aluminum molybdate and metal agglomeration.
To avoid this problem it has been proposed in, for example, U.S. Patent Application Publication No. 2008/0249342, regenerating a coked metal-containing methane dehydroaromatization catalyst by heating in a hydrogen-containing gas at a temperature of 700° C. to about 1200° C. so as to convert at least part of the carbonaceous material thereon to methane. However, although hydrogen regeneration is generally effective at removing freshly deposited coke while preserving metal dispersion, we have found that regeneration with hydrogen alone leads to a gradual build-up of graphitic coke on the exterior of the crystals of the zeolite support. This build-up eventually causes loss of access to the active sites of the catalyst and permanent deactivation of the catalyst.
In accordance with the present invention, it has now been found that regeneration in the presence of COx (CO and CO2) is an effective method of removing graphitic and other hard to remove coke, while preserving metal dispersion. The COx regeneration can be used alone or in combination with hydrogen regeneration. While this method is particularly effective in the regeneration of metal-containing methane dehydroaromatization catalysts, such as molybdenum-containing ZSM-5, it is believed to be equally applicable to other metal-containing catalysts, such as cobalt, tungsten, zinc, rhenium, platinum, palladium and mixtures thereof.
U.S. Patent Application Publication No. 2009/0305869 discloses a method of regenerating a ruthenium catalyst suitable for hydrogenation of aromatics, which comprises flushing the catalyst with inert gas in a regeneration step until the original activity or part of the original activity has been attained. The inert gas is selected from among nitrogen, carbon dioxide, helium, argon, neon and mixtures thereof and the flushing is carried out at a temperature of from 10 to 350° C.